The main purpose of a Neutral Grounding Resistor (NGR) is to limit the fault current present in a single line to ground fault. Limiting the possible current that can flow in such an event not only reduces damage to equipment but also helps safeguard personnel in the vicinity. Depending on the design of the resistor, this current value can be either high or low, continuous or of a predetermined duration. The resistor’s designed maximum current allows relays and protection settings to be easily determined and enacted, creating a safe work environment.
Resistance Grounding Comparison Chart – Standard Practices For Industrial Plants
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Voltage
600 volts and lower
2.4 to 35 kv
|
Grounding Method
Solid Grounding & High-Resistance Grounding
Low Resistance Grounding
|
|
Comparative Benchmarks for Various Grounding Methods
|
|
Methods of Grounding |
Characteristics |
Ungrounded |
Solid Grounded |
Low Resistance |
High Resistance |
Immunity to transient over-voltages |
Worst |
Good |
Good |
Best |
Increase in voltage stress under line-to-ground fault conditions |
Poor |
Best |
Good |
Poor |
Equipment protected against arc fault damage |
Worst |
Poor |
Better |
Best |
Safety to personal |
Worst |
Better |
Good |
Best |
Service reliability |
Worst |
Good |
Better |
Best |
Maintenance cost |
Worst |
Good |
Better |
Best |
Continued production after first ground fault |
Better |
Poor |
Poor |
Best |
Ease of locating first ground fault |
Worst |
Good |
Better |
Best |
Permits designer to coordinate protective devices |
Not Possible |
Good |
Better |
Best |
Ground fault protection can be added easily |
Worst |
Good |
Better |
Best |
Two voltage levels on the same system |
Not Possible |
Best |
Not Possible |
Not Possible |
Reduction in frequency of faults |
Worst |
Better |
Good |
Best |
First high ground fault current flows over grounding circuit |
Best |
Worst |
Good |
Better |
Potential flashover to ground |
Poor |
Worst |
Good |
Best |
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